The present invention relates to semiconductor fabrication and, in particular, to fabrication of container capacitors for dense semiconductor memory arrays.
Continuing advances in miniaturization and densification of integrated circuits have led to smaller areas available for devices such as transistors and capacitors. For example, in semiconductor manufacturing of a memory array for a dynamic random access memory (DRAM), each memory cell comprises a capacitor and a transistor. With shrinkage of the cell size, maintaining a sufficient amount of cell charge storage capacitance becomes a challenge in DRAM.
To increase capacitance, the semiconductor industry has moved from planar capacitor structures (e.g., xe2x80x9cparallel plate capacitorsxe2x80x9d) to vertical capacitor structures known as xe2x80x9ccontainer capacitorsxe2x80x9d. Several techniques have been developed to fabricate such capacitors. One such technique includes fabricating a cup-shaped bottom electrode defining an interior surface and an exterior surface formed on a substrate. A recess between adjacent bottom electrodes is formed in the insulating layer to expose a portion of the electrodes"" exterior surfaces. A capacitor dielectric and then a top electrode are deposited over the interior of the cup-shaped bottom electrode and the interior of the recess. Such a double-sided bottom electrode provides additional capacitance.
Conventionally, the bottom electrode of the double-sided electrode is formed of N-type hemispherical silicon grain (HSG). Using a double-sided HSG bottom electrode provides a higher surface area for increased capacitance. Current techniques to form the double-sided HSG bottom electrode include a selective HSG process and a combo HSG process. The selective HSG process requires selectively growing the HSG on the interior container surface and this results in an outside smooth and inside rough HSG electrode.
Poor selectivity of HSG growth results in HSG outgrowth on the exterior electrode surface, and this can cause cell-to-cell shorts, requiring the space between containers to be enlarged. The combo HSG process requires etching back the substrate using a hydrofluoric acid (HF) solution to expose a portion of the bottom electrodes outer surface. However, while etching back the substrate using the HF solution any pinholes present in the bottom electrode can cause locally preferential overetch and generate sinkholes and stringer problems in the substrate material. Further, the cell dielectric leakage increases due to the formation of high electric fields around sharp points due to a higher surface roughness of the HSG formed on the interior surface of the container. This results in a lower capacitance in the cell. Furthermore, current process flow to form the dual-sided HSG container exposes the formed HSG electrodes on a wafer surface to HSG floaters falling on the wafer and conductive surface defects that can cause cell-to-cell short.
Thus, there is a need in the art for a technique to form double-sided HSG electrodes that overcomes the above-described problems.
The present invention provides techniques for fabricating double-sided HSG electrodes for container capacitors that are more robust, less complex, and cost effective.
In one aspect, the invention provides methods for forming a double-sided HSG electrode. In one embodiment of the methods, the bottom electrode is fabricated by forming a layer of hemispherical silicon grain (HSG) polysilicon over interior surfaces of a container formed in a substrate. A barrier layer is then formed over the formed HSG polysilicon layer. Any HSG polysilicon and barrier layers formed outside and around the container opening during the formation of the HSG polysilicon and barrier layers is then removed to expose the substrate. A portion of outside surfaces of the formed HSG polysilicon is then exposed by removing the substrate, while the barrier layer is still on the interior surface of the container to prevent formation of sink holes and to prevent stringer problems during removal of the substrate. The barrier layer is then removed to expose the interior surfaces of the HSG polysilicon to form the double-sided HSG electrode.
In another aspect, the invention provides methods for forming an unsymmetrical cell nitride layer on a double-sided container electrode to improve cell capacitance and leakage performance. In one embodiment of the methods, a barrier layer over interior surfaces of a container is formed in a substrate. A bottom electrode is then fabricated by forming a layer of HSG polysilicon over the formed barrier layer such that the formed bottom electrode has a roughened interior surface and a smooth exterior surface. Any HSG polysilicon and barrier layers formed outside and around the container opening during the formation of the HSG polysilicon and barrier layers is then removed to expose the substrate. A portion of the barrier layer is then exposed by removing the substrate. A layer of nitride is then formed over the interior surface of the formed HSG polysilicon. The outside surface of the HSG polysilicon is then exposed by removing the exposed portion of the barrier layer. The formed nitride layer and the exposed outside surface of the HSG polysilicon is then pre-cleaned and a layer of cell nitrides deposited over the pre-cleaned surfaces. A top electrode is then formed over the deposited cell nitride layer to form the double-sided container electrode including unsymmetrical cell nitride layers.
In another aspect, the invention provides methods for forming a double-sided container electrode that reduces cell-to-cell shortage during process flow when the electrodes are on a wafer and exposed to process defects. In one embodiment of the methods, a bottom electrode is fabricated by forming a smooth polysilicon layer over interior surfaces of a container formed in a substrate. Any HSG polysilicon layer formed outside and around the container opening during the formation of the HSG polysilicon and barrier layers is then removed to expose the substrate. A portion of the substrate is then removed to expose the outside surface of the formed smooth polysilicon layer. A nitride layer is then deposited over the interior and exposed exterior surfaces of the smooth polysilicon layer. A barrier layer is then deposited over the nitride layer such that the barrier layer fills within and around the container. A recess is then formed to expose a top portion of the container by removing the barrier and nitride layers. Remaining barrier layer is then removed to expose the nitride layer. The exposed top portion of the smooth polysilicon is then oxidized to form an oxide on the top of the container. Remaining nitride layer is then removed to expose the smooth polysilicon surface of the container. A HSG polysilicon layer is then formed on the interior surfaces of the smooth polysilicon layer to form the dual-sided container electrode.
In another aspect, the invention provides methods for forming a double-sided container electrode that reduces cell-to-cell shortage during process flow when the electrodes are on a wafer and exposed to process defects. In one embodiment of the methods, a bottom electrode is fabricated by forming a smooth polysilicon layer over interior surfaces of a container formed in a substrate. Any HSG polysilicon layer formed outside and around the container opening during the formation of the HSG polysilicon and barrier layers is then removed to expose the substrate. A portion of the substrate is then removed to expose the outside surface of the formed smooth polysilicon layer. A barrier layer is then deposited over the dual-sided smooth polysilicon layer such that the barrier layer surrounds and fills the container. A portion of the formed barrier layer is then removed to form a recess such that the formed recess exposes a top potion of the formed dual-sided smooth polysilicon layer. The exposed top portion of the dual-sided smooth polysilicon layer is then nitridized to form a nitride cap. The remaining barrier layer is then removed to expose the polysilicon layer of the dual-sided bottom electrode. A HSG polysilicon layer is then formed over the interior surfaces of the smooth polysilicon layer within the container.
In another aspect, the invention provides a double-sided HSG electrode. In one embodiment, the double-sided HSG electrode includes a cup-shaped bottom electrode defining an interior surface and an exterior surface within a container formed in an insulative layer. The interior surface of the container comprises a HSG polysilicon layer and the exterior surface comprises a smooth polysilicon layer. A first dielectric layer overlies the interior surface of the lower electrode. A second dielectric layer overlies the first dielectric layer and the outer surface of the electrode, and a top electrode overlies the second dielectric layer.
In another aspect, the invention provides a double-sided HSG electrode. In one embodiment, the double-sided HSG electrode includes a cup-shaped bottom electrode defining an interior surface and an exterior surface within a container formed in an insulative layer. The interior surface of the container comprises a HSG polysilicon layer and the exterior surface comprises a smooth polysilicon layer. A top portion of the cup-shaped bottom electrode comprises an oxidized silicon cap to prevent cell-to-cell short. A dielectric layer overlies the lower electrode, and a top electrode comprising a conductive layer overlies the dielectric layer.
In yet another aspect, the invention provides a double-sided HSG electrode. In one embodiment, the double-sided HSG electrode includes a cup-shaped bottom electrode defining an interior surface and an exterior surface within a container formed in an insulative layer. The interior surface of the container comprises a HSG polysilicon layer and the exterior surface comprises a smooth polysilicon layer. A top portion of the cup-shaped bottom electrode comprises a nitride cap to prevent cell-to-cell short. A dielectric layer overlies the lower electrode, and a top electrode comprising a conductive layer overlies the dielectric layer.
Additional advantages and features of the present invention will be more apparent from the detailed description and accompanying drawings, which illustrate preferred embodiments of the invention.